1. Field of the Invention
The present invention relates to a fuel cell, and in particular relates to a solid polymer electrolyte fuel cell.
2. Description of the Related Art
In a fuel cell in which a polymer electrolyte is used, electric power and heat are simultaneously generated by an electrochemical reaction of a fuel gas containing hydrogen gas and an oxidizing gas containing oxygen gas or air. The fuel cell basically has a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes, that is, an anode electrode and a cathode electrode formed on both surfaces of the polymer electrolyte membrane. The electrode generally consists of a catalytic layer formed on a surface of the polymer electrolyte membrane layer and a gas-diffusion layer formed on an outside surface of the catalyst layer. The catalyst layer is mainly composed of a carbon powder supporting a platinum group metal catalyst. The gas-diffusion layer has a gas permeability and an electrical conductivity. In order to prevent leakage of the fuel gas and the oxidizing gas and mixing thereof externally, a gas seal member or a gasket is provided at the circumference of the electrode such that a polymer electrolyte membrane is held therebetween. The gas seal member or the gasket is assembled beforehand so as to be integrated with the polymer electrolyte membrane and the electrode. The assembly is referred to as a “Membrane Electrode Assembly” (hereinafter referred to simply as “MEA”). A conductive separator is disposed outside the MEA so as to mechanically secure MEA and electrically connect the adjacent MEAs in series or in parallel. A gas passage is formed at a contacting portion of the separator with the MEA for supplying reaction gases to an electrode surface and for collecting generated gases and excess gases. The gas passage can be provided separately from the separator. Generally, the separator has grooves as gas passages provided on a surface thereof.
In order to supply a fuel gas or an oxidizing gas to the above grooves, a pipe jig is required so as to branch pipes at the number of separators used and to directly connect the branched ends of the pipes to the grooves of the separators. The jig is referred to as a “manifold”. In particular, a type of manifold directly connecting a supply pipe of the above fuel gas or the above oxidizing gas is referred to as an “external manifold”. A type of manifold having a more compact design is referred to as an “internal manifold”. The internal manifold is constructed such that a through hole is provided to penetrate a separator in which gas passages are formed, an outlet and an inlet are communicated with the though hole, and a fuel gas or an oxidizing gas is directly supplied from the through hole. Since a fuel cell generates heat in operation, the fuel cell is required to be cooled by using cooling water, etc., so as to maintain desirable temperature conditions. In general, a cooling portion for flowing the cooling water at every one to three cells is provided so as to cool a fuel cell. There is a type of cooling portion disposed between adjacent separators and there is a type of cooling portion having a cooling water passage at a back surface of a separator. In particular, the latter type of the cooling portion is widely used.
A common stacked fuel cell is constructed such that a layered structure having 10 to 200 cells is produced by alternately layering MEAs, separators and cooling portions. The layered structure is held by end plates via a collecting plate and an insulating plate, and both ends of the end plates are fixed with fastening bolts. In the polymer electrolyte fuel cell, it is necessary that the separator have high conductivity, high air-tightness with respect to a fuel gas and an oxidizing gas, and high corrosion resistance in reactions of oxidizing or reducing hydrogen gas or oxygen gas. For this reason, a conventional separator was commonly composed of a carbon material such as a glassy carbon or an expansion carbon. Gas passages were produced by machining a surface thereof. In the case of the expansion carbon, gas passages were produced by forming with a mold. However, in the case of using the above materials, a process is used in which the above materials have high density in order to prevent gas leaks. Since the above materials are brittle, it is difficult to work the above materials, and the working cost is thereby increased. These problems have contributed to the high cost of fuel cells.
In recent years, in order to reduce this cost, various techniques for fuel cells were proposed. For example, a technique was proposed in Japanese Unexamined Patent Application Publication No. 8-180883, in which a separator is obtained by pressing or punching working on a metal plate as a separator material. However, in the case of obtaining the separator by performing only the above working method, collecting resistance of the fuel cell is increased due to corrosion of the separator, and durability thereof is decreased.
In order to solve the above problems, a conductive separator plate was proposed in Japanese Unexamined Patent Application Publication No. 2002-270196, in which a conductive separator plate is composed of a stainless steel having a corrugated surface, and a surface thereof is covered with a passivation film including enriched Cr. However, in the technique in which the concentration of Cr at the surface of the above stainless steel is enriched, a hard passivation film is formed not only on a surface of a separator opposite to a fuel electrode (hereinafter referred to as “fuel electrode side separator”) but also on a surface of a separator opposite to an oxidation electrode (hereinafter referred to as “oxidation electrode side separator”) although oxidation resistance is sufficiently obtained, and contact resistance of the separator with a MEA is thereby excessively increased. Due to this, good output performance of the fuel cell cannot be obtained in initial performance.
A technique was proposed in Japanese Unexamined Patent Application Publication No. 2003-123783, in which a surface of an oxidation side separator is roughened and a passivation film is then formed, although concentration of Cr is not limited. However, in this technique, since a passivation film made of TiN, etc., is formed on a surface of the oxidation side separator, working cost is increased. Due to the above various problems in the conventional techniques, in particular, it is required to prevent a decrease in durability due to corrosion of the separator and a decrease in output performance due to excessive increase in contact resistance of the separator with the MEA.
In polymer electrolyte fuel cells, a cooling medium is circulated in a fuel cell in order to dissipate heat generated by electrochemical reactions. A cooling medium passage, in which through holes having the same shape are formed at the corresponding positions on layered surfaces of an anode separator and a cathode separator, is used as a circulating means of a cooling medium in a layered structure in which a unit fuel cell (hereinafter referred to as a “cell”) are layered. In this way, a potential difference is generated among the above though holes and the respective cooling medium passages provided on a separator surface or among cooling medium passages of the respective separators. Due to this, short circuits may occur between separators holding a membrane electrode assembly (hereinafter referred to as “MEA”), and an electrical insulation property is required of the separator. A good insulating characteristic and a wide range temperature region (about −30 degrees C. to 150 degrees C.) are required for using the cooling medium. In particular, in fuel cells for automobiles, a pure water diluent such as an ethylene glycol is used in view of antifreeze characteristics, although it is desirable to use pure water.
In the above fuel cell technique, for example, a method is disclosed in Japanese Unexamined Patent Application Publication No. 7-230818, in which in order to secure an insulation property of a cooling medium of a solid polymer electrolyte fuel cell, after anode separators and cathode separators are stacked to obtain a stacked structure, a film forming material (sol in which tetraethoxysilane is hydrolyzed) is poured into a cooling medium passage and a though hole, and is adhered to surfaces thereof, so that a film is formed thereon. However, in the above technique, when a film is formed on the cooling medium passage by a sol-gel method, a hardening reaction occurs due to heating nitrogen gas, and growth of grains of the film is insufficient, whereby it is difficult to harden the film sufficiently. The heating temperature may be increased in order to improve hardening reaction of the film, but the separator may be deformed or deteriorated in this case.
It is necessary to use a heat exchanger such as a radiator which has a highly efficient cooling performance for cooling a fuel cell since the cooling medium is heated to 80 to 100 degrees C. in a solid polymer electrolyte fuel cell. In a conventional fuel cell system, in order to prevent formation of a short circuit via the cooling medium, a two-step cooling method is used in which the cooling medium supplied to an inside of a fuel cell stack is cooled by using a long life coolant circulating system (radiator cooling) via a counter-flow cooling apparatus. In particular, in metal separators, since contamination by impurity ions into the cooling medium possibly occurs due to the corrosion of the separator, it is necessary to control the conductivity thereof via an ion-exchange resin when the fuel cell is mounted in an automobile. Due to this, the system is complicated and is large, so that development of a one-step cooling system having good efficiency was required. However, in order to realize the one-step cooling system, it is necessary that the cooling medium circulation system be subjected to insulation coating treatment in order to prevent formation of the above short circuit and liquid junctions. When a film forming means by common insulating coating or common sol-gel method is used as the above coating treatment, heat conductivity of the separator is not sufficiently obtained, and heat exchange efficiency of the separator is decreased.
In the case in which a cooling medium surface is covered with a insulating coating, when the overall cooling medium surface is covered therewith, electrical conduction is prevented in a fuel cell layered direction. Due to this, in order to decrease electrical resistance as much as possible and to improve efficiency of the fuel cell, it is desirable that only the portion of the cooling medium surface which the cooling medium contacts be covered with insulating coating. However, in the case in which only the cooling medium passage is covered with the insulating coating, the separator is expensive.
In addition, for example, another method was proposed in Japanese Unexamined Patent Application Publication No. 2001-297784, in which a special material is disposed at a portion which contacts cooling water which circulates in a cell. In the material, ions in the cooling water are absorbed or discharged while applying voltage thereto, and voltage generated in the layered cell is applied thereto, so that there is a small potential difference between the cooling water and the cell in contact therewith, and component materials of the separator, etc., are not released and corroded in the cooling water. However, in the above technique, the apparatus has a complicated structure, and it is necessary to use expensive members. As a surface treating method in which stainless steel is used for a separator, a technique was proposed in Japanese Unexamined Patent Application Publication No. 2002-270196, in which a passivation film covering a surface of a stainless steel plate is controlled so as to contain Cr at 25 to 80 mass %. However, the above technique is related to a fuel gas passage and an oxidizing gas passage formed on a power generation surface of the separator, and thereby does not correspond to a cooling medium passage formed on a cooling medium surface of the separator.